<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .btn-group .active {
      background-color: orange;
      color: #fff
    }
  </style>
</head>

<body>
  <div class="result">3123</div>
  <div class="input-group">
    <input type="text" class="input1" value="0" />
    <input type="text" class="input2" value="0" />
  </div>
  <div class="btn-group">
    <button class="active" data-field="plus">+</button>
    <button data-field="minus">-</button>
    <button data-field="mul">*</button>
    <button data-field="div">/</button>
  </div>
  <script src="demoEs5.js"></script>
</body>

</html>